How to stop "Rogue" Particles
Gun Turret -



Blend file - Gunturret.blend

Not to take from Batfinger's comment above, this has been a common albeit minor problem for myself when emitting lines as particles.  Thanks to your
question I sat down and think I've cracked it at least for the gun projectile situation.



When using the generic "Line" particle for gun projectiles, reducing the emitter's vertices to just 1,  forces the launched lines along the correct trajectory.    There are no strays.  (Blender Vers 2.77a)

The emitter must be parented to the gun barrel if it's to be aimed or moved about.

Halo effect -  Rendered with Blender Internal



If using "Halo" in the Material slot, (e,g, for a space cannon) the emitter should be moved at least one line length from the end of the barrel.  When it's too close, the halo effect looks as though the plasma bolt (?) is being emitted from the turret and not launched from inside the barrel.

Emitter Object -

Using an Emitter Object for projectiles, comes up against the problem of rotating those previously launched when the barrel is aimed elsewhere.  They continue on their own trajectory but are pointing the wrong way!  Great for comedy!

Details -

The Particle plus Material settings are too numerous to list here but if you use the same as in the attached Blend file, none should stray from their intended trajectory.

Be mindful of BatFinger's mention about vertice normals.  Your single vertice may well be pointing elsewhere, not in line with the cannon.